Resolving Anharmonic Lattice Dynamics in Molecular Crystals with X-Ray Diffraction and Terahertz Spectroscopy
Martin Hutereau, Peter Banks, Ben Slater, J. Axel Zeitler, Andrew D. Bond, Michael T. Ruggiero
Abstract
Molecular crystals are increasingly being used for advanced applications, ranging from pharmaceutics to organic electronics, with their utility dictated by a combination of their three-dimensional structures and molecular dynamics-with anharmonicity in the low-frequency vibrations crucial to numerous bulk phenomena. Through the use of temperature-dependent x-ray diffraction and terahertz time-domain spectroscopy, the structures and dynamics of a pair of isomeric molecular crystals exhibiting nearly free rotation of a CF_{3} functional group at ambient conditions are fully characterized. Using a recently developed solid-state anharmonic vibrational correction, and applying it to a molecular crystal for the first time, the temperature-dependent spatial displacements of atoms along particular terahertz modes are obtained, and are found to be in excellent agreement with the experimental observations, including the assignment of a previously unexplained absorption feature in the low-frequency spectrum of one of the solids.